1. Field of the Invention
The present invention is related to ignition systems for internal combustion engines and more specifically to amplifier circuits which are used in conjunction with engine speedcrankshaft position sensors.
2. Description of the Prior Art
Over the past several years there has been a tendency in the automotive field to eliminate conventional mechanical breaker systems which provided timing pulses to ignition systems that, in turn, supplied properly timed spark energy to the individual spark plugs of the engine. One of the more popular replacements for the standard mechanical breaker system is the Hall effect sensor in combination with a rotating ferrous shunting element attached to the distributor shaft or the crank shaft of the engine. The Hall effect sensor is electrically connected to a solid state amplifier to produce a pulsating voltage signal to the primary of an ignition coil.
A typical sensor and circuit are shown in U.S. Pat. No. 3,875,920, whereby a multi-vaned shunt wheel is mounted in a distributor for rotation with the distributor shaft. As the shunt wheel is rotated, the individual ferrous shunting vanes rotate to a close proximity of the Hall effect sensor and a permanent magnet located next to the Hall effect sensor. The close proximity of any vane causes the magnetic field at the sensor to be reduced and thereby affect its electrical output signal. As the vane rotates past the Hall effect sensor, the magnetic field at the sensor increases and causes the electrical output signal from the sensor to increase. Therefore, a cyclical signal is generated by the Hall effect sensor, which is indicative of the speed at which the shunt wheel is rotating and is synchronized with the position. The amplifying circuit, as described in that patent, is used to drive a Schmitt trigger circuit when the output level of the Hall effect sensor rises above a certain predetermined level. Similarly, when the signal from the Hall effect sensor passes below another predetermined level, the output from the Schmitt trigger falls back to a low level.
Typically, the output of the Hall effect sensor is a cyclical signal having high and low peak values which are irregular and vary with respect to each other. In addition, the differential voltage derived from the Hall effect sensor provides an offset for the varying peak cyclical signal and this also may vary from sensor to sensor, depending upon the strength of the magnet, Hall voltage coefficient, and the value of the biasing voltage.
Variations in the peak to peak voltage are commonplace in Hall effect sensors since the vaned shunt wheels, such as shown in U.S. Pat. No. 3,875,920 and discussed above, are not precisely manufactured items and therefore may be either eccentrically mounted on the shaft or the vanes may have slight bends in them which cause variations in spacing between the individual vanes and the Hall effect sensor as the shunt wheel is rotated about the shaft.